Weldbonding, the process of joining materials with both welding and adhesive, offers numerous advantages over either welding or adhesive technology applied singly to materials joining. Among these advantages are stronger joints that reduce previous structural adhesive bonding limitations such as out-of-plane load-carrying capability, also commonly referred to as peel. Unfortunately, the development of weldbonding processes has been plagued with a number of problems.
The first problem has been the tendency of an adhesive to act as a non-conductive layer between the parts to be welded. One solution to this problem, that of welding only through areas that contain no adhesive, limits the placement of adhesive and substantially increases the labor required in accurate adhesive and weld placement. During manipulation, and particularly during heating, as will be discussed in detail below, adhesive originally placed outside the weld area tends to flow into the weld area, making techniques of excluding adhesive from the weld area problematic at best. Forming the parts to be welded to produce bosses or indentations that allow close material contact outside of an adhesive area can be used, but this requires additional tooling, and may be impractical depending on the design and characteristics of the materials to be joined. A more optimal method, such as that of the instant invention, would make it equally possible to weld through areas that contain adhesive and areas that do not contain adhesive. Such a method minimizes the effect of variations in adhesive application or flow of adhesive during the welding process.
Another approach has been to weld through the adhesive. An electrical shunt to obviate the insulating properties of the adhesive has been employed, as seen in U.S. Pat. No. 3,337,711 to Garscia. Adhesive additives to promote electrical and thermal conductivity; as seen in U.S. Pat. No. 6,146,488 to Okada, et al.; or that of U.S. Pat. No. 5,240,645 to Strecker; have also been used. However, the development of processes for welding through adhesives have themselves introduced complications unrelated to thermal or electrical conductivity that affect the quality of the weld.
The temperatures generated in the welding process produce a plurality of gases and particulates during the formation of the weld. Among these are vaporized elements from the materials themselves; vaporized coatings from the materials to be joined, which often vaporize at temperatures lower than that of the materials to which they are applied; and various vaporized materials and combustion products from the adhesive. In addition to rising towards the top of the weld pool, these materials tend to be expressed outwards through the weld area toward the periphery of the weld area, and their migration can both physically disrupt the weld and lead to inclusions within the weld that weaken the joint.
Various means have been employed in attempts to vent these gases and other byproducts from the weld area. For example, in U.S. Pat. No. 6,359,252 to Sanjeu, et al., an energy beam is directed to drive gaseous by-products away from the weld area. U.S. Pat. No. 5,859,402 to Maier teaches the use of a combined laser and arc welding method that utilizes a laser to remove dielectric material ahead of an arc. Such methods suffer the disadvantage of complexity in the necessity for utilizing secondary beams aside from the main welding technique.
Attempts have also been made to introduce physical spacers to allow a gas egress space between the materials to be joined, as seen in U.S. Pat. No. 4,682,002 to Delle Piane et al. In the first of several embodiments taught by Delle Piane '002, one of the materials to be joined is formed with a channel so that gases will have an egress route away from the weld area. In the second embodiment taught by Delle Piane '002, one or both of the materials to be joined is knurled in a pattern that provides a plurality of such passages. Both methods require machining or forming of the joined materials prior to weldbonding, which adds complexity to the process and may be impractical depending on the design and characteristics of the materials to be joined. The last embodiment taught by Delle Piane '002 involves clamping a formed spacer between the materials to be joined, in the vicinity of the weld area, to cause a physical separation of the materials to be joined and thereby to provide a means for gas egress. This technique requires the additional steps of placing the spacer, securing it during the welding process, and avoiding an obstruction of the venting channel by the securing process. This would be particularly likely when adhesive is used, as the method of Delle Piane '002 is not a weldbonding method and does not account for the potential obstruction to gas flow posed by adhesives in the weld area.
A more optimal method of achieving spacing between the materials to be joined in a weldbonding process is, as in the instant invention, to either incorporate inclusion bodies into the matrix of the adhesive, or alternately, to allow such inclusion bodies to be placed in the space between the materials to be joined along with the adhesive. Such inclusion bodies act to maintain a potential space for gas flow between the materials to be welded and minimize the opportunities for obstruction to gas flow.
This potential obstruction to gas flow due to adhesives in the weld area is a multi-faceted problem. In addition to the problems of gas formation in the area of the weld pool, the temperatures generated in welding processes affect the flow of adhesives in ways that compound the problem of gas generation from combustion of the adhesive. As the temperature in the weld area begins to rise, adhesive in the area surrounding the weld area also begins to rise in temperature. This tends to create a liquefying effect, whereby the viscosity of the adhesive in the vicinity of the weld area declines. The compressive force generated by holding the parts to be welded together therefore tends to press the increasingly less viscous adhesive towards the area of least adhesive density—the weld area. Additionally, the combustion or vaporization of metal, metal coatings, and adhesive tends to draw adhesive into the weld area. Accordingly, an undesirable cycle is created, by which combustion of the adhesive in the weld area tends to draw more adhesive into the weld area, which is in turn consumed in the high temperature weld pool, with the generation of more gas and gaseous by-products. The cycling of this process tends to increase the amount of gas and gaseous by-products produced and exacerbates the need for venting of the gases, lest the weld integrity become ever more degraded if these gases and gaseous byproducts escape through the weld pool. These problems are especially acute when a full penetration weld is not intended, as the gases are deprived of an egress route out the backside of the weld. Accordingly, an optimal adhesive will have sufficient resistance to flow to minimize flow into the weld area upon heating, but sufficient flow to allow gas egress through the adhesive matrix. An ideal material would be a relatively fast curing adhesive that could be substantially cured in the vicinity of the weld area by the welding process itself, yet still allow for gaseous egress.
Welding technologies using various heating phases are well known. However, such exemplars as U.S. Pat. Appl. No. 2002/0014476 to Tsukamoto, et al., and U.S. Pat. Appl. No. 2002/0079296 to Dijken, et al., which are not directed to weldbonding, teach phasing of the laser welding means only to optimize weld joint quality. An optimal phasing of heating cycles, especially in a laser welding context, would not only act to affect such parameters as weld flow and cooling rates, but would adjust the rates and duration of heating cycles so as to burn and remove, or vaporize, organics and other materials from the weld area, substantially cure the weldbonding adhesive in the vicinity of the weld area, and/or to optimally liquefy any weld enhancing additive that was contained in the adhesive or otherwise placed in proximity to the weld area. The utilization of a phase to vaporize or otherwise ablate, organics and other materials from the weld area prior to a welding phase results in improved weld quality.
Such weld enhancing additives, particularly those that are rich in silicon, can be added to weld areas of aluminum components to increase the quality of the weld. The high temperatures of welding tend to liquefy these additives, and they disperse into the weld pool. A recurrent problem has been to place these additives into the weld area, rather than outside the weld area where they would be of no use, and to insure that they are held in the weld area during heating long enough for them to be distributed into the weld pool. An optimal method, such as that of the instant invention, allows the weld additive or additives to be incorporated in the matrix of the adhesive, or alternatively and by way of example and not limitation, to be placed between the materials to be welded or placed otherwise on a surface of the material to be welded in the form of a powder, suspension, wire, tape, or foil.
What the art has needed has been a weldbonding method that allows welding to take place through an adhesive layer but does not require a layer of adhesive under a weld, allows gas and combustion products egress from the weld area without disruption or weakening of the weld and with minimal effect on the adhesive bonding, minimizes adhesive flow into the weld area to decrease the amount of adhesive combusted or vaporized, allows the introduction of weld enhancing additives to the weld area, and provides materials and processes that minimize the tendency of the weld to crack. The instant invention accomplishes these goals by utilizing a novel combination of methods and materials to utilize an adhesive and inclusion bodies that provide for a route of egress for gas and combustion products around the inclusion bodies and through the adhesive layer. Furthermore, the method is designed to minimize the amount of adhesive that is consumed or vaporized during the welding process by curing the adhesive sufficiently to minimize adhesive flow into the weld area during welding, thus minimizing the amount of gas and by-products that need to be vented from the weld area. The adhesive may also be enriched or otherwise utilized with various crack-reducing additives, including silicon enriched materials, that act as additives to improve the quality of the weld. Thus, a simplified method is utilized to minimize gas production, provide a means for venting such gases as are produced, and provide an optional additive element to improve the quality of the resulting weld. Lastly, a variation in welding technique of providing a multiphase heating pattern can be applied to maximize the benefits obtained with the present weldbond method.
With these capabilities taken into consideration, the instant invention addresses many of the shortcomings of the prior art and offers significant benefits heretofore unavailable.